1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image on a sheet, particularly to a copying machine, a printer and a facsimile machine.
2. Description of the Related Art
When an image is formed on both sides of a sheet in an image forming apparatus such as a copying machine, a printer and a facsimile machine, the image is initially formed on one surface (first surface) of the sheet and then the sheet is reversed such that a leading edge and a trailing edge of the sheet are countercharged to each other to be conveyed again. After a reversing conveyance of the sheet is carried out, the sheet passes through a two-sided conveyance path and an image is formed on another surface (second surface) of the sheet.
Japanese Patent Laid-open Publication No. 2002-292960 discusses such a configuration that, when the image is formed on the second surface of the sheet, a writing position in a main scanning direction (in a width direction intersecting with a conveying direction) of the image is corrected in the main scanning direction based on information from a position detection unit which is positioned in the two-sided conveyance path to detect a position of the sheet. Accordingly, positions of the images on the first surface and the second surface of the sheet are registered to each other.
In an electro-photographic color image forming apparatus, more specifically, in an apparatus in which toner images of respective colors are primary-transferred from a photosensitive drum to an intermediate transfer belt and the toner images of the respective colors which overlap one another in the secondary transfer unit are collectively transferred on the sheet, the position detection unit is provided in the two-sided conveyance path. This is because a distance between a first transfer unit and the second transfer unit is long in the color image forming apparatus and thus a position of the sheet is to be detected in an upstream side in the sheet conveying direction as far as possible when the toner images are transferred from the photosensitive drum to the intermediate transfer belt after the position detection unit detects the position of the sheet.
Further, Japanese Patent Laid-open Publication No. 09-100056 discusses an apparatus which using a reversing roller, performs the reverse-conveyance of the sheet for the purpose of correcting skew of the sheet having an image formed on the first surface. The skew of the sheet is corrected by bringing a leading edge of the sheet into contact with the reversing roller which stops rotation before the operation. Subsequently, the sheet is reversed by the reversing roller after the skew of the sheet is corrected and is conveyed to an image forming unit again in order to form another image on the second surface of the sheet.
In the two-sided conveyance path, when the position of the sheet in the width direction is detected by the position detection unit, it is material that the sheet is not skewed. It is because, if the sheet is skewed when the skew is corrected before forming the image, an error may arise in an accuracy of detection of the position of the sheet in the width direction by an amount of the skew. For example, if the another image is formed on the second surface based on sheet position information which is acquired from the detection unit and contains the error, misregistration occurs between an image position of the first surface and an image position of the second surface.
A certain effect can be produced to resolve the above described problem which arises when the position of the sheet in the width direction is detected by the position detection unit, if a technique is applied which brings the leading edge of the sheet into contact with the reversing roller in a rest position to correct skew of the sheet. However, when the sheet is reversed, rollers other than the reversing roller do not contact the sheet and thus the sheet is conveyed in a forward and backward direction only by using the reversing roller. Therefore, when the sheet is reversed by the reversing roller, the skew-feeding of sheet tends to occur. Since the position detection unit detects the position of the sheet in the width direction while the skew-feeding occurs when the sheet is reversed by the reversing roller, the accuracy in detecting the position of the sheet is reduced.